The Art market is a real bitch. Even if you know the provenance, and the artist is known, finding out recent auction prices is very hard to find out unless your prepared to pay various art forum's membership fees. (These are very expensive.)
If you've got an auction house nearby it might be an idea to give them a call, see if you can bring it down and have a chat. Do not sell it on ebay, ebay is for buying, not selling art, you'll only get a fraction of what it's worth.
Googling San Francisco, Verna and cartoonist gave me this. If you think it's the same person you might want to contact them.